Substrate cracking in Ti-6Al-4V driven by pulsed laser irradiation and oxidation

Oxide layers grown on Ti-6Al-4V using pulsed laser irradiation to thicknesses between 100 and 150 nm result in a variation in color of the films, but also exhibit a two-dimensional network of cracks (i.e. mudflat cracking). The crack densities increase with laser scan rate. Focused ion beam sections showed that the cracks not only occur within the oxide film (through thickness) but also penetrate into the titanium alloy substrate to depths between 1 and 6 μm. With this information and the elastic and fracture properties of the film and the substrate measured using nanoindentation, the residual tensile stress and strain undergone by the oxide film while cooling after the application of a laser pulse was estimated; this is the same stress that is relieved through cracking. The possibility that the fracture toughness of the substrate has decreased by embrittlement is also discussed.